270 CHEMICAL ENGINEERING
until it is filled up with grains, the water passing through them and the screen. The screen is then rotated about the vertex of the triangle until the dewatered solids slide freely over those in the pocket formed by the device, and the angle of slide is noted. This angle should be laid out on a piece of cardboard which will form a full sized template for sawing out wooden strips, to which the screen cloth will be secured for support. Secure the piece of cardboard to a wall and fasten at the upper end a piece of chain cutting the slope line. Draw the lower end of the chain up into a catenary loop securing it at some lower point of the slope line. The triangle formed by the original angle of repose plus the catenary bulge in its hypotenuse forms the template referred to. Cut out several pieces by it and tack the screen cloth over them. The first material fed into the device fills up the caternary pocket after which the solid material slides over the first solids fed in, which form the filter bed. Any desired depth of filtering pocket can be secured or a series of pockets may be laid out increasing in depth from top to bottom.
Devices for raking or scraping solids out of shallow tanks used for crystallization, precipitation or settlement, employ endless chains with flights mounted upon them or flight-pushing devices. In salt making these rise over the bed of salt in the pan on the back stroke and dip into it on the forward or advancing stroke. In removing salt from grainers the rakes in the latest and best reciprocating remover are actuated by a long stroke hydraulic cylinder. By the use of this means of actuation the number of linkage parts are reduced to a minimum, quite a consideration with the corrosive conditions which exist in salt houses. The hydraulic device is especially adapted to simply operating, the rakes under the battery of grainer pipes in grainers. The discharge ends of the salt pans are inclined and the rakes drag the salt up the slope and out of the pan.
The separation of solids from liquids often involves at the same time the separation of solids from solids. In the so-called "classifiers" much used in Western ore mills the separation effected is watery sands from watery fines or slimes. The water content in the sands is of course very much less than in the slimes, the latter containing the bulk of the water fed to these devices. To reduce the water content in the slimes following their passage out of the classifiers they are often passed through thickening devices and the thick discharge from these is subjected to mechanical filter treatment and drying for further moisture reduction.
In the simplest forms of separation apparatus employing settlement with or without overflow of a product containing fines the settled material if sufficiently fine to pass a small opening can be drawn off continuously through a plug. This mode of disposition involves much escape of liquid and dilution of the discharging material and methods of avoiding this will be discussed at a later point in this section. If the material in the separating vessel is being elutriated as in preparing size from size stock means for discharging a charge of the material being extracted will be provided at the bottom of the separating vessel, the washing containing the substance being extracted being removed by solution and siphoning.
Mechanical Classifiers.—These devices consist of a tank into which the material fed flows at one end and what does not settle in the tank overflows at the lower end. Mechanical means are provided for removing the settled material. These devices have attained much use in ore mills of late years but others of similar